To treat or not to treat: CUBN-associated persistent proteinuria.

Background: Persistent proteinuria is an important indicator of kidney damage and requires active evaluation and intervention. However, tubular proteinuria of genetic origin typically does not improve with immunosuppression or antiproteinuric treatment. Recently, defects in CUBN were found to cause isolated proteinuria (mainly albuminuria) due to defective tubular albumin reuptake. Unlike most other genetically caused persistent albuminuria, CUBN C-terminal variants have a benign course without progression to chronic kidney disease according to the literature. Here, we present Korean cases with persistent proteinuria associated with C-terminal variants of CUBN.

Methods: We identified Korean patients with CUBN variants among those with an identified genetic cause of proteinuria and evaluated their clinical features and clinical course. We also reviewed the literature on CUBN-associated isolated proteinuria published to date and compared it with Korean patients.

Results: All patients presented with incidentally found, asymptomatic isolated proteinuria at a median age of 5 years. The proteinuria was in the subnephrotic range and did not significantly change over time, regardless of renin- angiotensin system inhibition. Initial physical examination, laboratory findings, and kidney biopsy results, when available, were unremarkable other than significant proteinuria. All patients maintained kidney function throughout the follow-up duration. All patients had at least one splicing mutation, and most of the variants were located C-terminal side of the gene.

Conclusion: We report Korean experience of CUBN-related benign proteinuria, that aligns with previous reports, indicating that this condition should be considered in cases with incidentally found asymptomatic isolated proteinuria, especially in young children.